When a scene in the object field of an optical imaging system is imaged onto a focal plane, specular reflections (i.e., light from a natural source such as the sun, or from a man-made source such as an illuminating laser, that is reflected from surfaces within the object field of the system) can appear as glints in the focal plane.
A "specular reflection" occurs when an incident electromagnetic wave is reflected from a surface in the object field of the optical imaging system in a definite direction so that the directions of the incident and reflected waves make equal angles with (and lie in the same plane as) a line perpendicular to the reflecting surface. Glints are defined as bright images of unresolved objects that occur randomly in the focal plane of the optical imaging system. Specular reflections usually emanate from regions of the object field that are unresolved by the optical imaging system, and normally appear as point sources (i.e., Airy disk patterns) in the focal plane of the system.
In the case of an active optical tracking system, a high-energy laser (HEL) beam is used to illuminate a target (e.g., a hostile missile). Specular reflections of the HEL beam from the target can appear as glints in the focal plane of the tracking system. Regions of the object field of the tracking system that are unresolved by the system could include, e.g., integral portions of a target that is being tracked, background clutter, and debris resulting from countermeasures against the actively-tracked target.
Reflections of an HEL beam from an actively-tracked target typically radiate in relatively small solid angles. Thus, glints occurring in the focal plane of an active optical tracking system are likely to contain relatively large energy fluxes. A large number of such glints can produce a total energy flux sufficient to mask the image of the target in the focal plane, or at least to fuzz the image of the target in the focal plane to a significant extent. In general, glints in the focal plane tend to impede imaging. In an active optical tracking system, glints can hinder accurate pointing of the HEL beam toward a selected target during crucial phases of the target's trajectory.
The problem of optically distinguishing a diffuse object in the presence of very bright sources of optical noise has been encountered in the prior art in connection with, e.g., image-forming and image-processing systems. In an article by P. F. Mueller and H. J. Caulfield entitled "Photographic Dynamic Range Suppression", Applied Optics, Vol. 19, (1980), pages 4134-4135, a technique called "dynamic range compression" (also called "unsharp masking") for eliminating glare in photographs is discussed, (where "glare" is defined as a very bright extended region of reflection).
Other optical image processing techniques that have been used in the prior art for detecting objects in a variety of applications (e.g., earth resource studies, meteorology, automatic surveillance and/or inspection, pattern recognition, and bandwidth compression) include matched filtering; correlation processing; edge detection; image subtraction; and inverse filtering using various kinds of holographic elements, pupil plane masks and gratings in coherent or incoherent systems. However, the optical detection of an object in the presence of very bright point-source images in the focal plane presents some unusual difficulties that have not heretofore been addressed in the design of real-time imaging and image-processing systems.
A major difficulty in optically detecting an object such as an actively tracked target involves the determination of glint positions in the focal plane of the tracking system. In the somewhat related discipline of optical pattern recognition and detection, target positions in the focal plane can be determined by techniques such as matched filtering, or optical correlation, or image subtraction. However, matched filtering and optical correlation techniques are extremely sensitive to parameters such as rotation, size, shape and orientation of the target, and hence are of only limited usefulness in tracking targets (e.g., hostile missiles) that cannot be completely characterized a priori with respect to such parameters.
Image subtraction techniques, on the other hand, are relatively insensitive to the rotation, size, shape and orientation of a target, and hence in principle could be used in determining glint positions in target tracking applications. A helpful discussion of image subtraction techniques is found in an article by J. F. Ebersole entitled "Optical Image Subtraction", Optical Engineering, Vol. 14, (1975), pages 436-447. However, until the present invention, no practicable method had been developed for implementing an image subtraction technique for use in determining glint positions (or otherwise suppressing glints) in the focal plane of an optical imaging system.